Experts Puzzle Over What Happened to Missing Plane

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This picture taken from aboard a flying Soviet-made AN-26 used as a search aircraft by Vietnamese Air Force to look for missing Malaysia Airlines flight MH370, shows an officer (R) and a reporter looking out the window during search operations over the southern seas off Vietnam on March 9, 2014. Malaysia on March 9 launched a terror probe into the disappearance of a Malaysian Airlines passenger jet carrying 239 people the day before, investigating suspect passengers who boarded with stolen passports, as relatives begged for news of their loved ones. The United States sent the FBI to investigate after Malaysia Airlines flight MH370 vanished from radar early on March 8 somewhere at sea between Malaysia and Vietnam, but stressed there was no evidence of terrorism yet. AFP PHOTO / HOANG DINH NAM (Photo credit should read HOANG DINH NAM/AFP/Getty Images)

What happened to Malaysia Airlines Flight 370, which vanished Saturday over the South China Sea en route to Beijing? Former U.S. transportation officials are stumped. Here are some key theories that investigators are considering as they struggle to understand how a usually reliable wide-body jet could apparently disappear from a clear sky — with no distress call and no apparent debris field, either. A mid-air mechanical malfunction would likely have caused either a debris field or allowed time for a distress call. An explosion would likely leave debris, too, and a bomb is unlikely given the target. Maybe a hijacking? That seems unlikely given security — and what would have happened to the plane thereafter? Pilot error or sabotage could also be possible, in theory — but an emergency call should still have been possible in that case. Click through to read more about some of the possible fates experts are considering for the missing plane.